Generally, miniature silicon condenser microphones such as capacitive MEMS, micro-electrical-mechanical system, microphones can be integrated with CMOS circuitry and can be produced by batch processing which makes the device to device variation much smaller. Further, miniature silicon condenser microphones have a form factor compatible with state-of-the-art hand held device architectures, and do not need an electret for electrical biasing which makes them easier to solder onto a printed circuit board, PCB, than conventional electret microphones.
However, it is difficult to achieve high sensitivity and low noise with very small volumes. Further, a conventional silicon capacitive microphone always needs through-wafer etching to create separation of the membrane and back plate layer, which is a time consuming and costly process.
Typically a condenser microphone system may consist of four elements; a fixed, perforated back plate, a highly compliant, moveable membrane or diaphragm (which together form the two plates of a variable air-gap capacitor), a voltage bias source, and a buffer amplifier.
FIG. 1 shows a basic structure of such a capacitive microphone 1 (left) and three possible implementations on a silicon wafer 5 (right). The microphone 1 comprises a diaphragm or membrane 2, a back plate 3, and a back chamber 4. The back chamber 4 illustrated in the left-hand drawing of FIG. 1 may be implemented in the chip-package or on the wafer5.
The membrane 2 should be highly compliant and precisely positioned relative to the back plate 3, while the back plate should remain stationary and present a minimum of resistance to the flow of air through it. Achieving all of these characteristics in microphones below 1 mm in size using integrated circuit materials is rather challenging. Typical stress levels in integrated circuit thin films, if not relieved in the finished diaphragm or membrane 2, are many times greater than the levels at which the diaphragm becomes unusable due to over-stiffening or buckling. Compliance tends to decrease very rapidly with decreasing size for a given diaphragm material and thickness.